I lost my mum to suidide 2 years ago @suchclearwater - I know you're not yet sure that's the case with your sister, but either way I'm so so sorry you are going through this. it's awful. Almost impossible to see how you'll ever recover at the time. But I promise, you can and you will.
Right now having children to think of and manage just feels like another burden, but my daughter absolutely saved me - having to keep going for her gave me something to hang on to when I was going under. Hold your children close, just being with them and loving them will keep you connected to your old life and help keep you afloat.
I want to tell you not to feel guilty. But I know it's so much easier said than done. I still feel like I could or should have done more to prevent my mother dying. I dream about it. But something that clicked with me in counselling was going through all the things she had going on at the time she chose to die and realising how small my part was in that. It's hard not to see the thing you could have done as being potentially transformative; but realistically, no one person can completely turn another's life around. You could never have protected her from everything, or shepherded her through every step of recovery. Her life was out of your hands.
I really strongly recommend that you try and get some counselling as soon as you feel able to (took me almost 4 months, I was so caught up in sorting everything out - I left it too late, I was really not very well by the time I got help).
Please be kind to yourself. Don't feel like you have to keep soldiering on with life as usual. You don't. Something terrible has happened. Lots of people have to go through grief, and it doesn't get accorded enough time and respect; but losing someone like this has a lot of terrible trauma attached to it, a lot of the usual comforting thoughts won't apply, and you need time to find your own way through the uniquely awful feelings it throws up.
I'm so sorry about your sister 